Filters are known for filtration at different particle size levels. Filtration at a very fine particle size level is required for example in the treatment of beer for clarification, in the treatment of chemical effluent and in particular in the treatment of water to remove, for example, bacteria.
Many conventional membrane filtration processes for fine particle size filtration of liquids rely on high velocity cross flow to induce turbulence in the feed and reduce the thickness of the laminar flow boundary layer along the membrane surface. In this way, the rate at which the "filter cake" builds up is reduced, and the transmembrane flux may be kept higher than would otherwise be the case. Nevertheless, the typical transmembrane flux to be expected for any form of membrane when used for real feeds is .about.100 l/hr/m.sup.1 /Bar, compared with pure water flux of a typical polymer of organic microfiltration membrane in the range .about.1000-10000 l/hr/m.sup.2 /Bar. Therefore, not only are all types of microfiltration membranes used at &lt;10% of their theoretical efficiency, a lot of power input is wasted in doing it.
The direct membrane cleaning process developed by AEA Technology, Harwell, and described in GB 2160545 relies on the generation of hydrogen bubbles at the surface of an electrically-conducting, cathodically-charged, porous membrane to break up the filter cake. In this way, it is possible to maintain transmembrane fluxes approaching the value to be expected for pure water. Inorganic membranes typically cost at least five times as much as organic membranes. If the transmembrane flux for an inorganic membrane can be increased significantly by use of this process, not only does it become more cost effective, because the filtration area will be much smaller, but the running costs may also be less because high cross flow velocities may not be required.
The process of GB 2160545 uses sintered metal membranes. These have pore sizes in the range of 1 micrometre, so do not reject bacteria and such like micro-organisms and cannot be used for many critical processes of commercial interest. Carbon membranes can be obtained with a wide range of pore sizes. However, as most are in the form of tubes, they would need either a coaxial wire anode, or an external tube anode to be used in the electrolytic process. These membranes tend to be flexible and so would have to be short to maintain the correct anode-cathode distance. Also, it would be much more difficult to develop good enough electrical contact. These factors would increase considerably the engineering cost of such a filter module.
One known membrane which is made by the Ultram Company comprises a flat, sintered-metal substrate with a pore size of approximately one micrometer and, with a smaller pore size, ceramic, final filtration layer. The latter is usually kept as thin as possible because the materials from which it is made are expensive. Nevertheless, the small pore sizes are required for the membrane to reject bacteria. It is known to clean this filter by the use of back pressure.